Development of a roll-to-roll high-speed laser micro processing machine for preparing through-holed anodes and cathodes of lithium-ion batteries

Aiming to improve the battery performance of lithium-ion batteries (LIBs), modification of the cathodes and anodes of LIBs using laser beams to prepare through-holes, non-through-holes or ditches arranged in grid and line patterns has been proposed by many researchers and engineers. In this study, a laser processing system attached to rollers, which realizes this modification without large changes in the present mass-production system, was developed. The laser system apparatus comprises roll-to-roll equipment and laser equipment. The roll-to-roll equipment mainly consists of a hollow cylinder with openings on its circumferential surface. Cathode and anode electrodes for LIBs are wound around the cylinder in the longitudinal direction of the electrodes. A pulsed beam reflected from the central axis of the cylinder can continuously open a large number of through-holes in the thin electrodes. Through-holes were formed at a rate of 100 000 holes per second on lithium iron phosphate cathodes and graphite anodes with this system. The through-holed cathodes and anodes prepared with this system exhibited higher C-rate performance than nontreated cathodes and anodes.


Introduction
Lithium-ion batteries (LIBs), as mobile power sources, have expanded our world by realizing safe and portable equipment that can be used for long periods with a single charge [1]. The performance requirements of LIBs are increasing as their applications expand. To meet performance demands, many researchers and engineers around the world are competing with each other for the purpose of improving performance [2]. In the improvement of LIB performance, although the development of materials that exhibit higher battery performance has received attention [3], the cell arrangement of LIBs also determines LIB performance [4][5][6]. Recently, threedimensional laser processing of cathode and anode layers has been reported [7][8][9][10][11][12][13][14]. The main reason for applying laser technology to LIBs is the increase in the number of pathways for Li + transfer to/from particle surfaces of the cathode and anode active materials through the sidewalls of throughholes [15,16]. In addition, through-holed layers have cracks and voids formed by laser irradiation. The cracks and voids have improved the high-rate performance because the electrolyte solution directly contacts the active material particle surfaces, which increases the interaction and deintercalation of Li + ions to/from these surfaces [17]. There are many papers in which the three-dimensional laser processing of cathode and anode layers improved the remarkable high-rate performance of LIBs [18][19][20][21][22][23]. Not only high-rate performance but also energy density [24] can be improved by making throughholes in cathode and anode layers with laser technology. Specifically, at 25 • C, while cells composed of nontreated Lithium iron phosphate (LFP) cathodes and graphite anodes exhibited a discharge capacity retention of 0% at 10 • C after 250 cycles, a cell composed of through-holed LFP cathodes and graphite anodes exhibited a capacity retention of 76% at 10 • C after 250 cycles [15]. In the charge/discharge cycle test at 2 • C, through-holed and nontreated electrodes exhibited discharge capacity retention of 79% and 20%, respectively. A through-holed structure can improve the lifetime of cells [15]. In addition, through-holed anode structures could increase the energy density of a cell because prelithiation of anodes in cells containing through-holed cathodes and anodes laminated via separators can avoid the irreversible capacity that appears when an anode is first charged [25].
As explained above, laser processing of the surfaces of cathode and anode layers can improve the battery performance, such as by providing a high-rate input/output of current and an improved energy density. However, the implementation of laser technology would adversely affect current LIB mass production. Recently, the price of LIBs has been one of the important factors determining the permeation of LIBs into our daily life. Because complications in LIB mass production cause an increase in the price of LIBs, the implementation of laser technology in LIB mass production should be done without any change in mass production lines or any increase in price. The roll-to-roll system is used to prepare thin cathode and anode electrodes in the current manufacturing process for commercially available LIBs. The current speed of delivering cathode and anode electrodes in the roll-to-roll system is 2-5 m min −1 . Holing, through-holing or ditch formation should be conducted periodically at the speed of electrode delivery with sufficient reproducibility in cathodes and anodes. As previously shown by us, a through-holed structure with a hole diameter of 20 µm and an opening ratio (the opening ratio is the area of the hole with respect to the electrode area when the electrode is viewed from the front) of 5%-10%, in which the holes have a grid pattern, was prepared [15,16,24,25]. The through-holes could be produced on cathodes and anodes with 120 mm width and 50 µm thickness with the developed laser system attached to the roll-to-roll system. The system provides high-speed through-holing on cathodes and anodes. Through-holing has produced remarkable improvements in the performance of cathodes and anodes, such as enhancement of charge/discharge performance at high current density [16], elimination of low-rate performance caused by the difference in capacity between both sides of a current collector foil [26] and elimination of the irreversible capacity observed on anodes during the first charging process [25]. In this paper, the design and performance of a laser processing system are presented, and holes prepared on cathodes and anodes with the laser processing system for mass production of LIBs are characterized and explained. To prepare uniform holes without burrs of current collectors and build throughholed structures at high speed, cathode and anode foils are fed to a roll on which they move helically, and then laser beams are emitted from the center of the roll to the cathode and anode foils through narrow openings on the surface of the roll (figure 1). As explained below, through-holes that are arranged in a staggered pattern can be formed at a higher speed than conventional methods by adjusting the transfer speed of the cathode and anode electrodes on the roller and the intensity and pulse width of the laser beam. The structures we prepared with the high-speed laser micro processing system were characterized, and the battery performance of the prepared through-holed electrodes for LIBs is reported below.

Background
To open through-holes on cathode and anode electrodes with long width and thin thickness by laser processing, in general, a laser beam reflected by a rotating polygon mirror or galvanometer mirror scans the surface of a thin electrode to form holes in the matrix. An f-theta ( fθ) lens is then used to form an image on the thin electrode, and the thin electrode is scanned at a constant speed by a laser beam reflected at a constant angular speed by a rotating polygon mirror. However, if the diameter of the beam incident to the fθ lens is increased, the laser beam is blurred on the cathode or anode, and conversely, if the diameter of the beam incident to the fθ lens is decreased, the diameter of the beam on the cathode or anode is increased. Additionally, in a case where the focal length is increased to reduce the swing angle of the fθ lens, the diameter of the beam is increased, and it is difficult to form uniform through-holes. Furthermore, in such a method, the power density of the laser beam varies depending on the processing portion. Therefore, burrs are formed around through-holes or non-through-holes are formed because excessively high or low power is applied. Our laser system developed in this study can solve the existing problems of polygon mirrors or galvanometer mirrors as described above. The object of our development is to provide a laser processing apparatus with which through-holes can be formed continuously with high accuracy and at high speed.

System design
The laser processing apparatus of our laser system has a hollow cylinder with openings on its circumferential surface around which the cathode or anode electrode to be processed is wound obliquely in the longitudinal direction of the electrode (figure 2). A motor with a rotation axis arranged coaxially with the central axis of the cylinder, a reflecting member fixed to a rotating shaft of the motor, and a pulsed laser beam work together to make through-holes on the cathode and anode electrodes. The pulsed beam enters from the direction of the rotation axis and then is reflected in the radial direction of the cylinder by the reflecting member; thus, the electrodes are irradiated with the pulsed beam through the openings to create through-holes (figure 3), thereby solving the above-described problems.
The intensity and pulse width of the pulsed beam are adjusted to open through-holes in the thin electrodes with single pulse irradiation. In addition, the rotation speed of the mirror reflecting the pulsed beam in the radial direction of the cylinder and the transfer speed of the thin electrodes on the cylinder should be synchronized to pass the pulsed beam to the opening on the cylinder. The through-holes are arranged in a staggered array. The hole diameter and distance between holes can be controlled by the rotation speed of the cylinder and laser power (figure 4). A video introducing the system developed in this study can be found in the supporting information section.
To remove foreign substrates (metal fumes from current collectors and impurity products from the cathode and anode active materials) that are formed in the laser through-holing process, which degrade the battery performance and then cause the thermal runaway of LIBs, a vacuum system for dust collection was used in the laser system. Because foreign substrates were formed near the opening of the cylinder, two vacuum suction ports were set inside and outside of the cylinder. The collection of foreign substrates caused no damage to the cathode and anode as a result of peeling the cathode and anode layers from current collectors.

Through-holed samples
To confirm the performance of our laser system, aluminum (Al, thickness 15 µm) and copper (Cu, 15 µm) foils and polyphenylene sulfide (PPS, 25 µm) and polyimide (PI, 25 µm) films with 120 mm width were through-holed with the laser system using a green (532 nm) nanosecond pulsed laser. The foils and films were transferred onto the cylinder at 2.6 m min −1 .

System conditions for preparing through-holed cathodes and anodes
The laser used in this study was a nanosecond pulsed laser with a second harmonic wavelength of 532 nm, a maximum laser output (MLO) of 28 W and a maximum pulse repetition rate (MPRR) of 50 kHz (the pulse length (PL) was 15 ± 3 × 10 −9 s) for LFP cathodes. In the case of graphite anodes, MLO was 45 W, and MPRR was 200 kHz (PL was 15 ± 3 × 10 −9 s). In this study, a one-shot drilling approach was adopted to produce through-holes. The laser beam was condensed at the surface of the cathodes and anodes by an fθ lens ( f value: 100; the lens was originally made by the authors for this laser system). Here, the focal point was set on the top of the surface of the cathode and anode that were mounted on the cylinder. A total of 100 000 through-holes with a hole diameter of 20 µm could be formed per second on the LFP cathode and graphite anode electrodes with this laser processing system.    and anode active materials. Cathodes and anodes were prepared using the following protocol. In the preparation of LFP cathodes, LFP powder, acetylene black (AB, Denka Black, Denka Co., Ltd, Japan) as a conductive assistant material and polyvinylidene difluoride (PVDF, KF9130, Kureha Co., Japan) as a binder were added at a weight ratio of LFP:AB:PVDF = 86:7:7 in N-methyl-2-pyrrolidone and then mixed with a planetary mixer (Mazerustarm KURABO, Japan). The solid content of the slurry was 30%. The slurry had a suitable viscosity for forming a final active material layer with the desired uniform thickness on Al foils (thickness of 20 µm, JIS H4160 1N30-H18, Toyo Aluminum K.K., Japan). After the slurry was coated using a doctor blade, the slurry on the Al foils was dried at 80 • C for 1 h in vacuum to obtain LFP layers. The LFP layers were formed on both sides of the Al foils. The LFP layer/Al foil/LFP layer cathodes were pressed in a roll press machine; the obtained cathodes had a loading amount of 2.0 ± 0.1 mg cm −2 , and the thickness of the LFP layer formed on one side of the Al foil was 17 ± 1 µm. In the preparation of graphite anodes, graphite, vapor-grown carbon fiber (VGCF, VGCF ® -H, Showa Denko K.K., Japan) as a conductive assistant material, carboxymethyl cellulose (CMC, cat.# 6139, Polysciences Inc., USA) as a thickener, and styrene-butadiene rubber (SBR, S2910(J)-1, JSR Corporation, Japan) as a binder were used to form the anode slurry. Copper (Cu) foil (thickness: 10 µm, nonpolished, Furukawa Electric Co., Ltd, Japan) was used as a current collector. The solvent for the graphite slurry was water. The weight ratio of the materials in the graphite anodes was graphite:VGCF:CMC:SBR = 91:5:2:2. The solid content of the anode slurry was 36%. The same process used to prepare the LFP cathodes was used to prepare the graphite anodes. After pressing, the loading amount and thickness of the graphite anodes were 1.8 ± 0.1 mg cm −2 and 35 ± 1 µm on each side of the Cu foil, respectively.
To assess the performance of the prepared LFP cathodes and graphite anodes, cells in which a cathode or anode was paired with a lithium (Li) metal foil were prepared. A stack consisting of Li metal/separator film (Hipore™, thickness: 25 µm, Asahi Kasei Co., Japan)/through-holed (or nontreated) cathode (or anode)/separator film/Li metal foil was set into a laminate-type pack for LIBs. The size of the LFP and graphite electrodes in the cells was 35 mm × 25 mm. Electrolyte solution consisting of a mixture of EC:DMC = 1:1(v/v) + 1M LiPF 6 + vinylene carbonate (2.7 wt.%) (Ube Chemicals, Japan) was injected into the laminate-type pack. An electrolyte solution volume of 0.7 ml was used for each cell.
Constant-current (CC) mode with a cutoff voltage of 2.0-4.0 and 0.02-2.0 V (vs. Li/Li + ) for LFP cathodes and graphite anodes, respectively, was used in charge−discharge cycle tests using a battery charge/discharge controller (HJ1001SD8, Hokuto Denko Corporation, Japan). All experiments were performed at 25 • C with an incubator (MIR-154, Panasonic, Japan). An open-circuit period of 10 min was set before the charge and discharge process in all charge−discharge cycles. Degassing was not performed after the first charge−discharge cycle. The theoretical capacities (mAh g −1 ) of the LFP cathode and graphite anode active materials are 150 and 350 mA h g −1 , respectively. In different C-rate performance tests, the current for each C-rate was calculated using the theoretical capacities of the cathode and anode materials. Discharge rates of 0.1 • C, 0.2 • C, 0.5 • C, 1 • C, 2 • C, 3 • C, 5 • C and 10 • C were used for the LFP cathodes and graphite anodes. In the C-rate performance tests, LFP cathodes and graphite anodes were charged in CC/constant-voltage modes at 0.1 • C, and then, after an open-circuit period of 10 min, the discharge process was performed with different C rates in CC mode. To determine the discharge capacity retention at each C-rate, three cycles of the charge process at 0.1 • C and discharge process at different C-rates were examined. Three electrodes were produced in each experiment and used for evaluation. Data were averaged from the values obtained from three electrodes. In the graphs of the results, the error bars of each data point are shown, and the maximum and minimum values of the error bars are the maximum and minimum values among the three data points.

Chemical analysis of through-holed cathodes and anodes
A scanning electron microscope (SEM, SU-8010, Hitachi High-Technologies, Japan) equipped with ultrahigh-resolution semi-in-lens electron optics with a Super-ExB filter, an upper detector above the objective lens and a lower detector was used for microscopic observations of the surface morphologies of the samples. Simultaneously, energy-dispersive x-ray spectroscopy (EDX, X-Max, Horiba, Japan) was performed to determine the elemental compositions of the samples by detecting the emission of characteristic x-rays from the corresponding elements at an accelerating voltage of 15 kV. Cross sections of the cathodes and anodes were analyzed with a focused ion beam (FIB) coupled with SEM (FIB-SEM, MI4000L, Hitachi High-Technologies, Japan) to assess the shape of the through-holes.
The electron states in the anode and cathode materials were evaluated with x-ray photoelectron spectroscopy (XPS, JP-9010 MC, JEOL, Japan). A MgKα x-ray source was used. The binding energy for all XPS spectra was corrected based on the binding energy of C (1s). Holes with a relatively uniform size were arranged on the foils and films in a 'staggered array'. The target hole diameter on the Al and Cu foils was 7 µm. This target value was selected because it was uncertain whether uniform holes with a diameter of 7 µm could be consistently formed. The target and measured through-hole diameters and through-hole intervals on the foils and films are listed in table 1. Almost all throughholes were arranged as desired and had the shape of a perfect circle with the appropriate diameter. The diameters of the through-holes were higher than the target values. This discrepancy was attributed to the method of this laser system, which punches through-holes with one shot of a powerful laser. In particular, in the case of polymer films, hole diameters larger than the target diameter were observed because polymer films are vulnerable to laser beams. The transfer rate of foils and films with a width of 120 mm on the cylinder was 2.6 m min −1 . The formation speed of holes was 100 000 holes per second. The transfer rate of foils and films is much smaller than that used in the through-holing process of LFP cathodes and graphite anodes. Figure 6 shows SEM images of the surface of through-holed LFP cathodes observed with (a), (c) low and (b), (d) high magnifications. As mentioned above, the through-holes were arranged in squares in a 'staggered array' (e). The interval between adjacent holes was 180 µm. The average hole diameter, which is the smallest diameter observed in the highmagnification surface SEM images, and the hole diameter at  the position of the current collectors were estimated to be 11.7 µm on the side of the laser incidence plane (a), (b). The average inlet diameter of the holes at the surface of the LFP layer was 20 µm. From the difference between the diameters at the inlet and current collector, the holes had a tapered shape on the side of laser incidence plane. On the other hand, on the side of the laser emission plane (c), (d), a difference between the two diameters was not observed. In contrast to the holes on the side of the laser incidence plane, the shape of the holes on the laser emission side was a cylinder. In addition, in the LFP cathodes, discoloration could be seen around the holes on both sides of the laser incidence and emission planes. The reason for the discoloration is due to the tapered shape of the holes in the laser incidence plane and the bulging of the LFP layer around the through-holes in the laser emission plane. It was attributed to debris scraped out by passage of the laser beam. Minor cracks were also seen around the through-holes in the laser emission plane.

Hole size and consistency in through-holed LFP cathodes and graphite anodes
In the case of the graphite anodes (figure 7), the intervals between adjacent holes were 180 and 315 µm. Additionally, in the graphite anodes, a tapered shape could be observed on the laser incidence side. The average hole diameter at the position of the current collector was 5 µm on the side of the laser incidence plane (a), (b). The average inlet hole diameter at the surface of the graphite layer was 38 µm. In the preparation of through-holed graphite anodes, the power of the laser was decreased because the amount of graphite powder blown away by the incident laser beam in this system was higher than that in the system with a rotating polygon mirror or galvanometer mirror. Therefore, the diameters of holes at the position of the current collector were much smaller than those of the through-holed LFP cathode prepared with the previous system. Nevertheless, the inlet diameters at the surface of the graphite layer were still much larger than that of the throughholed LFP cathode prepared with the present laser processing system. The shape of the holes on the side of the laser emission plane (c), (d) was almost as straight as the holes formed on the side of the laser emission plane of the LFP cathode layer. The standard deviation of the data for holes in the laser emission plane could not be evaluated because the hole openings had various shapes, and it was not possible to define the opening diameter. However, very irregular hole diameters in figure 7(c) were observed. Opening ratios of 0.2%-0.4% and 0.05%-0.1% were obtained for the LFP cathode and graphite anode, respectively. The percentage of graphite powder lost from the graphite anodes was 8 wt.% based on the difference in weight between graphite layers before and after throughholing. The distance between the lines of holes was 315 µm (e). One small agglomerate can be seen in the SEM image of figure 7(a). The agglomerate was observed at this frequency. Cross-sectional SEM images of a through-holed (a) LFP cathode and (b) graphite anode are shown in figure 8. From the SEM images, the formation of through-holes by one shot of the laser beam was confirmed. In addition, as mentioned above, the degree of taper of the through-holes of graphite anodes is much higher than that of LFP cathodes. The reason why no LFP material is visible on top of the current collector in figure 8(a-1) is that the powder LFP material was blown away by the incident laser beam. The tapers of the Al and Cu current collectors in the through-holed LFP cathode and graphite anode were almost the same in both directions of the laser incidence and emission planes. The reason is currently under investigation.

Chemical and mechanical analysis of through-holed cathodes and anodes
The through-holing process with our present laser system was performed in air. Therefore, Al and Cu, which are structural materials of current collectors, can be considered to be redeposited after evaporation by laser irradiation. In addition, the irradiated cathode and anode active materials are expected to be oxidized or burned during the through-holing process. The EDX mapping in figure 8 shows the distribution of Al and Cu atoms on cross sections of a through-holed LFP cathode and graphite anode. When compared with the through-holed anode and cathode prepared in our previous study with a laser processing system based on a rotating polygon mirror or galvanometer mirror [17], Al and Cu atoms were not redeposited on the inner wall of the holes on the side of the laser emission plane in the present laser system. Conversely, for through-holes prepared with our previous laser processing system, Al and Cu atoms were distributed in the holes on the laser incidence side because the tips of the holes grow and advance little by little with each weak shot of the laser beam [17]. The present laser system uses a single laser shot to penetrate the anode and cathode layers and current collector. Therefore, Al and Cu atoms are not scattered on the inner wall of holes that are formed in the laser travel direction.
To assess the chemical changes in the cathode and anode materials after through-holing, XPS spectra of the LFP cathode and graphite anode were collected before and after through-holing (figure 9). XPS spectra of a (1) non-laser-treated and (2, 3) through-holed LFP (a-d) cathode and graphite anode (e) were measured. In addition, spectra were collected on the sides of the (2) laser incidence and (3) emission planes so that the difference in chemical states between two planes of the cathode and anode could be determined. The XPS spectra obtained from the laser incidence plane are identical to those obtained with the nontreated LFP cathode and graphite anode except for Li 1s. The Li 1s XPS spectrum of the nontreated LFP cathode extends to the high binding energy side, indicating that the nontreated LFP cathode contains lithium compounds on the LFP particle surfaces. Laser incidence might remove lithium compounds from the LFP particle surfaces. The XPS spectra of the laser emission plane exhibited a peak shift to the high binding energy side. It can be seen that the LFP particle surfaces on the laser emission plane are oxidized to some extent by laser irradiation. Additionally, in the case of graphite anodes, the C1s XPS spectrum of the laser emission side extends to the high binding energy side. The oxidation of LFP and graphite particle surfaces, which occurred on the laser emission side of the LFP cathode and graphite anode, may decrease the capacity of the electrodes. The change in capacity caused by laser incidence to the cathode and anode should be confirmed by analyzing the performance of the LFP cathode and graphite anode.
Through-holing of cathodes and anodes might cause a decrease in the mechanical strength of the cathode and anode layers. To compare the difference in mechanical strength between nontreated and through-holed graphite anodes (figure 10), (a) nontreated and (b) through-holed graphite anodes were doubled over to (1) crease the surface and then restored to their original state (2,3). The creased parts of the anodes were carefully observed to check for the formation of cracks, splitting and peeling. Cracks, splitting, and peeling were not observed on the creased parts of both (a-2, a-3) nontreated and (b-2, b-3) through-holed graphite anodes. The photographs indicate that the mechanical strength of the through-holed graphite anode is identical to that of the nontreated graphite anode. SEM images of creased graphite anodes ((a-4) and (b-4)) show cracks at the creased parts. Nontreated and through-holed graphite anodes did not have any difference in the creased parts. Agglomerates can be observed around the cracks in the SEM images of both (a-4) and (b-4). Because agglomerates could  not be observed on the surfaces of the graphite anodes before creasing, the agglomerates were considered to be formed with the cracks. From the results of SEM-EDX analysis, it was found that the main component of the agglomerates was carbon. Therefore, in the SEM images, a difference in mechanical strength between nontreated and through-holed graphite anodes was not observed. Tensile strength tests are currently under investigation.

Performance of through-holed and nontreated cathodes and anodes
Considering the weight loss of the cathode and anode materials by laser incidence, the through-holed cathode and anode exhibited the same capacities of 150 and 350 mAh g −1 at 0.1 • C as determined by charging/discharging tests of the nontreated cathode and anode. Judging from the measured capacities of the through-holed and nontreated cathodes and anodes, the cathode and anode materials were not damaged by laser irradiation, although changes in the chemical states of the cathode and anode active materials can be inferred from the XPS spectra. Figure 11 shows the plots of discharge capacity retention vs. C-rate with through-holed and nontreated cathodes and anodes at 25 • C at C-rates of 0.1, 0.2, 0.5, 1, 2, 3, 5 and 10. The discharge rate performance of the through-holed cathodes and anodes is much better than that of the nontreated cathodes and anodes as well as that of through-holed cathodes and anodes fabricated with a laser processing system using a rotating polygon or galvanometer mirror reported in our previous papers [15,16,24]. Specifically, the discharge capacity retention observed with the through-holed LFP cathode and graphite anode fabricated with the laser system developed in this study are 92.3% and 66.0% at 10 • C, respectively. In the through-holed electrodes fabricated with our previous laser, the discharge capacity retention of the LFP cathode and graphite anode was 93% and 68%, respectively [27]. The effect of the three-dimensional structure formed in the cathode and anode by our present laser processing system on the transfer of Li + ions is identical to that obtained with the laser system in which a rotating polygon mirror or galvanometer mirror is used.

Conclusions
In this study, a laser processing device for forming throughholes on cathodes and anodes for LIBs was developed to enable instantaneous introduction of the device into conventional mass-production processes and suppress manufacturing costs for LIBs. Specifically, cathodes and anodes delivered in the roll-to-roll system are through-holed with laser beams. To produce regular holes arranged at constant designed intervals, cathodes and anodes were transferred onto a hollow cylinder with openings. A pulsed laser beam entered from the center of the cylinder and passed through the openings, and then cathodes and anodes were through-holed with one shot of laser irradiation. Metal foils, polymer films and electrodes with a width of 120 mm can be delivered on the roll-to-roll system at a delivery rate of 2.6 m min −1 . Regular through-holes could be formed continuously on the entire surface of metal foils, polymer films and electrodes delivered on the roll-to-roll system. Through-holes with a diameter of 12 and 5 µm were uniformly arranged with a distance between holes of 180 and 315 µm on cathode and anode electrodes, respectively. In the chemical and mechanical tests of through-holed electrodes, no residual metal pieces due to laser processing were observed, and no chemical changes in the materials were observed. Because there was a concern that the mechanical strength of the electrode would decrease due to laser processing, tests in which through-holed and nontreated electrodes were folded and stretched were conducted. Observation of the folded portions revealed no difference in electrode surface between through-holed and nontreated electrodes. Based on these results, it was concluded that the durability of the through-holed electrodes was not greatly reduced by laser processing.
Comparing cells fabricated with through-holed electrodes and with conventional nontreated electrodes, it could be confirmed that the three-dimensional structure significantly improved the high-rate performance because the throughholing structure could create many pathways in the active material layers through which Li + ions could access the particle surfaces of active materials.
For the laser processing device developed and presented here, the electrodes are limited to a maximum width of 150 mm. A roll-to-roll system for our laser processing system that can deliver electrodes with a width of 1000 mm has been developed. Some basic tests to verify that the roll-to-roll system and laser system were functioning were performed at a delivery rate of 5 m min −1 . In addition, if the laser output can be increased, the delivery rate (e.g. the through-holing rate) can be increased. Currently, efforts to increase the delivery rate while maintaining the regularity of hole shape and arrangement are in progress.